diff --git a/dom/base/nsCopySupport.cpp b/dom/base/nsCopySupport.cpp
index 98e0e1c9247d..70662e93e07c 100644
--- a/dom/base/nsCopySupport.cpp
+++ b/dom/base/nsCopySupport.cpp
@@ -845,8 +845,16 @@ nsCopySupport::FireClipboardEvent(EventMessage aEventMessage,
   // all stored data, before we return.
   auto clearAfter = MakeScopeExit([&] {
     if (clipboardData) {
-      clipboardData->SetMode(DataTransfer::Mode::Protected);
-      clipboardData->ClearAll();
+      clipboardData->Disconnect();
+
+      // NOTE: Disconnect may not actually clear the DataTransfer if the
+      // dom.events.dataTransfer.protected.enabled pref is not on, so we make
+      // sure we clear here, as not clearing could provide the DataTransfer
+      // access to information from the system clipboard at an arbitrary point
+      // in the future.
+      if (originalEventMessage == ePaste) {
+        clipboardData->ClearAll();
+      }
     }
   });
 
diff --git a/dom/events/DataTransfer.cpp b/dom/events/DataTransfer.cpp
index 1f87c3c905a3..39bf5779cbb2 100644
--- a/dom/events/DataTransfer.cpp
+++ b/dom/events/DataTransfer.cpp
@@ -81,6 +81,23 @@ enum CustomClipboardTypeId {
   eCustomClipboardTypeId_String
 };
 
+// The dom.events.dataTransfer.protected.enabled preference controls whether or
+// not the `protected` dataTransfer state is enabled. If the `protected`
+// dataTransfer stae is disabled, then the DataTransfer will be read-only
+// whenever it should be protected, and will not be disconnected after a drag
+// event is completed.
+static bool
+PrefProtected()
+{
+  static bool sInitialized = false;
+  static bool sValue = false;
+  if (!sInitialized) {
+    sInitialized = true;
+    Preferences::AddBoolVarCache(&sValue, "dom.events.dataTransfer.protected.enabled");
+  }
+  return sValue;
+}
+
 static DataTransfer::Mode
 ModeForEvent(EventMessage aEventMessage)
 {
@@ -98,7 +115,9 @@ ModeForEvent(EventMessage aEventMessage)
     // the DataTransfer, rather than just the type information.
     return DataTransfer::Mode::ReadOnly;
   default:
-    return DataTransfer::Mode::Protected;
+    return PrefProtected()
+      ? DataTransfer::Mode::Protected
+      : DataTransfer::Mode::ReadOnly;
   }
 }
 
@@ -1276,6 +1295,15 @@ DataTransfer::ConvertFromVariant(nsIVariant* aVariant,
   return true;
 }
 
+void
+DataTransfer::Disconnect()
+{
+  SetMode(Mode::Protected);
+  if (PrefProtected()) {
+    ClearAll();
+  }
+}
+
 void
 DataTransfer::ClearAll()
 {
@@ -1608,5 +1636,15 @@ DataTransfer::FillInExternalCustomTypes(nsIVariant* aData, uint32_t aIndex,
   } while (type != eCustomClipboardTypeId_None);
 }
 
+void
+DataTransfer::SetMode(DataTransfer::Mode aMode)
+{
+  if (!PrefProtected() && aMode == Mode::Protected) {
+    mMode = Mode::ReadOnly;
+  } else {
+    mMode = aMode;
+  }
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/events/DataTransfer.h b/dom/events/DataTransfer.h
index 6dc787a67aa4..837d5156026d 100644
--- a/dom/events/DataTransfer.h
+++ b/dom/events/DataTransfer.h
@@ -229,9 +229,7 @@ public:
   Mode GetMode() const {
     return mMode;
   }
-  void SetMode(Mode aMode) {
-    mMode = aMode;
-  }
+  void SetMode(Mode aMode);
 
   // Helper method. Is true if the DataTransfer's mode is ReadOnly or Protected,
   // which means that the DataTransfer cannot be modified.
@@ -272,6 +270,12 @@ public:
                           nsISupports** aSupports,
                           uint32_t* aLength) const;
 
+  // Disconnects the DataTransfer from the Drag Data Store. If the
+  // dom.dataTransfer.disconnect pref is enabled, this will clear the
+  // DataTransfer and set it to the `Protected` state, otherwise this method is
+  // a no-op.
+  void Disconnect();
+
   // clears all of the data
   void ClearAll();
 
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index 63e6df0f3962..b94dc0c88e01 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -1774,8 +1774,7 @@ EventStateManager::GenerateDragGesture(nsPresContext* aPresContext,
         new DataTransfer(window, eDragStart, false, -1);
       auto protectDataTransfer = MakeScopeExit([&] {
         if (dataTransfer) {
-          dataTransfer->SetMode(DataTransfer::Mode::Protected);
-          dataTransfer->ClearAll();
+          dataTransfer->Disconnect();
         }
       });
 
diff --git a/dom/events/test/test_bug508479.html b/dom/events/test/test_bug508479.html
index a2be834abcc4..9cf9e22b8a1d 100644
--- a/dom/events/test/test_bug508479.html
+++ b/dom/events/test/test_bug508479.html
@@ -74,7 +74,7 @@ function runTests()
 
   SpecialPowers.addChromeEventListener("drop", chromeListener, true, false);
   var targetNotHandling = document.getElementById("nothandling_target");
-  fireDrop(targetNotHandling, false, true);
+  fireDrop(targetNotHandling, true, true);
   SpecialPowers.removeChromeEventListener("drop", chromeListener, true);
   ok(chromeGotEvent, "Chrome should have got drop event!");
   is(gGotHandlingDrop, false, "Didn't get drop on accepting element (2)");
diff --git a/dom/tests/mochitest/general/test_clipboard_events.html b/dom/tests/mochitest/general/test_clipboard_events.html
index dd8df0af9c72..8b7f4a4d9705 100644
--- a/dom/tests/mochitest/general/test_clipboard_events.html
+++ b/dom/tests/mochitest/general/test_clipboard_events.html
@@ -746,7 +746,13 @@ function test_image_dataTransfer() {
   }
 }
 
-SimpleTest.waitForFocus(doTests);
+SimpleTest.waitForFocus(() => {
+  SpecialPowers.pushPrefEnv({
+    // NOTE: These tests operate under the assumption that the protected mode of
+    // DataTransfer is enabled.
+    "set": [["dom.events.dataTransfer.protected.enabled", true]]
+  }, doTests);
+});
 
 </script>
 </pre>
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index d6cfe70e2c96..64bd5c771f4d 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -8246,8 +8246,7 @@ PresShell::HandleEventInternal(WidgetEvent* aEvent,
       // made protected, and then disconnected.
       DataTransfer* dataTransfer = aEvent->AsDragEvent()->mDataTransfer;
       if (dataTransfer) {
-        dataTransfer->SetMode(DataTransfer::Mode::Protected);
-        dataTransfer->ClearAll();
+        dataTransfer->Disconnect();
       }
       break;
     }